Is it saying that if you plug in 2 numbers (x + y) into the function f, the value returned will be what would be returned by the same function if you plugged in each number separately ( f(x) and f(y) )?

I also don't understand what f(1) = 3 is saying. Is it stating that I'm plugging in 1 for x + y, so that (x + y) = 1, which then gets sent to the function for calculations?

What's more important to me is getting at the source of my confusion, so that I can solve this on my own.

Is it saying that if you plug in 2 numbers (x + y) into the function f, the value returned will be what would be returned by the same function if you plugged in each number separately ( f(x) and f(y) )?

Yes. In particular, since you have not been provided with any formal rule (that is, a formula) for the function, you must use the one value provided, being "f(1) = 3", to find other functional values by creating sums.

For instance, since 2 = 1 + 1 and since f(1 + 1) = f(1) + f(1), then you can find the value of f(2).